The present invention is related to a method of compression molding plastics
Present practice in compression molding neat plastics and plastic composites is to feed preheated sheets of the plastic material into a press, close the press and simultaneously deform the hot plastic sheet into the desired component shape while cooling it. Since the plastic in contact with the mold cools quickly, the forces needed to deform the plastic can be enormous, particularly for large area components.
Another problem with flow forming plastics and particularly plastic components in compression molding is achieving smooth surfaces As soon as the plastic sheet blank enters the mold it begins cooling, even before the press closes and therefore before the plastic has been molded. This rapid cooling not only increases molding pressures required to flow form the plastic, but also gives rise to surface roughness. When composite materials are used, the rapid cooling results in exposed fibers, porous areas, and visible boundaries of the sheet blank between resin frozen at the surface and the newly formed resin areas produced when molten resin in the composite sheet interior moves to the surface. Changing the mold temperature greatly increases cycle time since the mold requires large masses of metal to contain the stresses of the molding operation and these large masses take time to heat and cool.
It is an object of the present invention to provide a method of compression molding thermoplastic sheets which reduces the required molding pressure and therefore reduces press size, which is particularly significant in large part fabrication.
It is another object of the present invention to provide a method of compression molding which results in finished products with smooth surfaces.
It is a further object of the present invention to provide a method of compression molding thermoplastic parts which maintains short cycle times and therefore high throughput for each press.
It is another object of this invention to provide a means for transporting plastic sheet blanks into the mold.
It is yet another object of the present invention to provide a method of compression molding which results in reduced thermal decomposition of the thermoplastic resins.